Hydrodynamic depressor for marine sensor streamer arrays

ABSTRACT

A hydrodynamic foil system for a sensor streamer array includes at least two tow ropes each coupled at one end to a survey vessel and at the other end to a paravane. the paravanes configured to provide lateral outward force as the vessel and paravanes are moved through a body of water. A spreader cable is coupled to each of the paravanes. A plurality of laterally spaced apart sensor streamers coupled at forward ends thereof to the spreader cable. A plurality of foils is disposed on the spreader cables. The foils configured to provide hydrodynamic lift in a vertical direction as the spreader cables are moved through the water.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH, OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the field of marine geophysicalsurveying. More particularly, the invention relates to devices forcontrolling depth of the forward end of sensor streamers.

2. Background Art

Marine geophysical surveying includes seismic surveying systems. Seismicsurvey systems are used to acquire seismic data from Earth formationsbelow the bottom of a body of water, such as a lake or the ocean. Marineseismic surveying systems typically include a seismic vessel havingonboard navigation, seismic energy source control, and data recordingequipment. The seismic vessel is typically configured to tow one or morestreamers through the water. At selected times, the seismic energysource control equipment causes one or more seismic energy sources(which may be towed in the water by the seismic vessel or by anothervessel) to actuate. Signals produced by various sensors on the one ormore streamers are ultimately conducted to the recording equipment,where a record with respect to time is made of the signals produced byeach sensor (or groups of such sensors). The recorded signals are laterinterpreted to infer the structure and composition of the Earthformations below the bottom of the body of water.

The one or more streamers are in the most general sense long cables thathave seismic sensors disposed at spaced apart positions along the lengthof the cables. A typical streamer can extend behind the seismic vesselfor several kilometers. Because of the great length of the typicalstreamer, the streamer may not travel entirely in a straight line behindthe seismic vessel at every point along its length due to interaction ofthe streamer with the water and currents in the water, among otherfactors.

More recently, marine seismic acquisition systems have been designedthat include a plurality of such streamers towed by the seismic vesselin parallel. The streamers are towed by the vessel using towing devices,and associated equipment that maintain the streamers at selected lateraldistances from each other as they are towed through the water. Suchmultiple streamer systems are used in what are known as threedimensional and four dimensional seismic surveys. A four dimensionalseismic survey is a three dimensional survey over a same area of theEarth's subsurface repeated at selected times. The individual streamersin such systems are affected by the same forces that affect a singlestreamer.

The quality of images of the Earth's subsurface produced from threedimensional seismic surveys is affected by how well the positions of theindividual sensors on the streamers are controlled. The quality ofimages generated from the seismic signals also depends to an extent onthe relative positions of the seismic receivers being maintainedthroughout the seismic survey. Various devices are known in the art forpositioning streamers laterally and/or at a selected depth below thewater surface. U.S. Pat. No. 5,443,027 issued to Owsley et al., forexample, describes a lateral force device for displacing a towedunderwater acoustic cable that provides displacement in the horizontaland vertical directions. The device has a hollow spool and arotationally mounted winged fuselage. The hollow spool is mounted on acable with cable elements passing therethrough. The winged fuselage ismade with the top half relatively positively buoyant and the bottom halfrelatively negatively buoyant. The winged fuselage is mounted about thehollow spool with clearance to allow rotation of the winged fuselage.The difference in buoyancy between the upper and lower fuselagemaintains the device in the correct operating position. Wings on thefuselage are angled to provide lift in the desired direction as thewinged fuselage is towed through the water. The device disclosed in theOwsley et al. patent provides no active control of direction or depth ofthe streamer, however.

U.S. Pat. No. 6,011,752 issued to Ambs et al. describes a seismicstreamer position control module having a body with a first end and asecond end and a bore therethrough from the first end to the second endfor receiving a seismic streamer. The module has at least one controlsurface, and at least one recess in which is initially disposed the atleast one control surface. The at least one control surface is movablyconnected to the body for movement from and into the at least one recessand for movement, when extended from the body, for attitude adjustment.Generally, the device described in the Ambs et al. patent is somewhatlarger diameter, even when closed, than the streamer to which it isaffixed, and such diameter may become an issue when deploying andretrieving streamers from the water.

U.S. Pat. No. 6,144,342 issued to Bertheas et al. describes a method forcontrolling the navigation of a towed seismic streamer using “birds”affixable to the exterior of the streamer. The birds are equipped withvariable-incidence wings and are rotatably fixed onto the streamer.Through a differential action, the wings allow the birds to be turnedabout the longitudinal axis of the streamer so that a hydrodynamic forceoriented in any given direction about the longitudinal axis of thestreamer is obtained. Power and control signals are transmitted betweenthe streamer and the bird by rotary transformers. The bird is fixed tothe streamer by a bore closed by a cover. The bird can be detachedautomatically as the streamer is raised so that the streamer can bewound freely onto a drum. The disclosed method purportedly allows thefull control of the deformation, immersion and heading of the streamer.

It is also important to control the depth of the streamers in the waterso that effects of seismic signal reflection from the water-airinterface can be controlled. There exists a need for devices to controlthe depth of the forward end of a streamer in a streamer array in thewater.

While the explanation of the need for the invention is generallyexplained in terms of seismic surveying, it is important to recognizethat the invention is applicable to any survey system which includes aplurality of laterally spaced apart sensor streamers towed by a vessel.Such other types of streamers may include, without limitation,electrodes, magnetometers and temperature sensors. Accordingly, theinvention is not limited in scope to seismic streamers.

SUMMARY OF THE INVENTION

One aspect of the invention is a hydrodynamic foil system for a sensorstreamer array that includes at least two tow ropes each coupled at oneend to a survey vessel and at the other end to a paravane. The paravanesare configured to provide lateral outward force as the vessel andparavanes are moved through a body of water. A spreader cable is coupledto each of the paravanes. A plurality of laterally spaced apart sensorstreamers coupled at forward ends thereof to the spreader cable. Aplurality of foils is disposed on each spreader cable. The foils areconfigured to provide hydrodynamic lift in a vertical direction as thespreader cables are moved through the water.

A method for operating a data acquisition system according to anotheraspect of the invention includes towing at least two tow ropes from aseismic survey vessel in a body of water. A distal end of each tow ropehas a paravane coupled thereto. A lateral separation between theparavanes is maintained using a spreader cable coupled to each paravane.A forward end of a plurality of sensor streamers at laterally spacedapart positions is coupled to each spreader cable. A selected verticalforce is applied to each spreader cable along its length to move therespective spreader cable to a selected depth in the body of water.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a marine seismic acquisition system according to oneembodiment of the invention.

FIG. 2 shows one example of a hydrodynamic foil couple to a spreadercable in the system of FIG. 1.

FIG. 3 shows a plurality of the foils of FIG. 2 affixed to a spreaderrope as in FIG. 1.

DETAILED DESCRIPTION

FIG. 1 shows an example marine geophysical survey system that caninclude a plurality of sensor streamers laterally spaced from eachother. Each of the sensor streamers can be guided through the water byone or more lateral force and depth (“LFD”) control devicescooperatively engaged with each of the streamers, although the LFDdevices are not required to be used in the present invention. Thegeophysical survey survey system includes a survey vessel 10 that movesalong the surface of a body of water 11 such as a lake or the ocean. Thevessel 10 may include thereon equipment, shown at 12 and for conveniencecollectively called a “recording system.” The recording system 12typically includes a recording unit for making a record with respect totime of signals generated by various seismic sensors in the acquisitionsystem. The recording system 12 also typically includes navigationequipment to determine at any time the position of the vessel 10 andeach of a plurality of sensors 22 disposed at spaced apart locations onstreamers 20 towed by the vessel 10. The foregoing elements of therecording system 12 are familiar to those skilled in the art and are notshown separately in the figures herein for clarity of the illustration.

The sensors 22, for example, can be any type of seismic sensor known inthe art such as motion responsive sensors, acceleration sensors,pressure sensors, pressure time gradient sensors or any combinationthereof. Seismic sensors measure seismic energy primarily reflected fromvarious structures in the Earth's subsurface below the bottom of thewater 11. The seismic energy may originates from a seismic energy source(not shown) deployed in the water 11. The seismic energy source (notshown) may be towed in the water 11 by the seismic vessel 10 or adifferent vessel (not shown). The recording system 12 may also includeseismic energy source control equipment (not shown separately). In othernon-limiting examples, the sensors 22 may be electromagnetic sensorssuch as electrodes, wire loops or coils or magnetometers, and the energysource (not shown) may be an electromagnetic transmitter.

In the survey system shown in FIG. 1, there are four sensor streamers 20towed by the seismic vessel 10. The number of sensor streamers may bedifferent in any particular implementation of a survey system accordingto the various aspects of the invention, therefore, the number ofstreamers such as shown in FIG. 1 is not intended to limit the scope ofthe invention.

As explained in the Background section herein, in acquisition systemssuch as shown in FIG. 1 that include a plurality of laterally spacedapart streamers, the streamers 20 are coupled to towing equipment thatsecures the forward ends of the streamers 20 at selected lateralpositions with respect to each other and with respect to the seismicvessel 10. As shown in FIG. 1, the towing equipment can include twoparavane tow ropes 8 or cables each coupled to the vessel 10 at one endthrough a winch 19 or similar spooling device that enables changing thedeployed length of each paravane tow rope 8. As used herein, “tow rope”is intended to mean any generally spoolable device to transmit axialtension and can include fiber rope, armored cable or any similar devicecombination of devices for such purpose. The distal end of each paravanetow rope 8 is functionally coupled to a paravane 14. The paravanes 14are each shaped to provide a lateral component of motion to the varioustowing components deployed in the water 11 when the paravanes 14 aremoved through the water 11. “Lateral” in the present context meanstransverse to the direction of motion of the vessel 10. The lateralmotion component of each paravane 14 is opposed to that of the otherparavane 14, and is generally in a direction transverse to thecenterline of the vessel 10 and thus its direction of motion. Thecombined lateral motion of the paravanes 14 separates the paravanes 14from each other until they place the components of the survey system inselected lateral positions. In one example, the separation is selectedto place into tension one or more spreader ropes or cables 24. Thespreader ropes of cables may be interconnected across the entire spanbetween the paravanes 14, or in other example may be separated.

The streamers 20 are each coupled, at the axial end thereof nearest thevessel 10 (“forward end”), to a respective lead-in cable termination20A. The lead-in cable terminations 20A are coupled to or are associatedwith the spreader ropes or cables 24 so as to fix the lateral positionsof the streamers 20 with respect to each other and with respect to thevessel 10. Electrical and/or optical connection between the appropriatecomponents in the recording system 12 and, ultimately, the sensors 22(and/or other circuitry) in the ones of the streamers 20 inward of thelateral edges of the system may be made using inner lead-in cables 18,each of which terminates in a respective lead-in cable termination 20A.A lead-in termination 20A is disposed at the vessel end of each streamer20. Corresponding electrical and/or optical connection between theappropriate components of the recording unit 12 and the sensors in thelaterally outermost streamers 20 may be made through respective lead-interminations 20A, using outermost lead-in cables 16. Each of the innerlead-in cables 18 and outermost lead-in cables 16 may be deployed by arespective winch 19 or similar spooling device such that the deployedlength of each cable 16, 18 can be changed.

The spreader ropes or cables 24 may include thereon a plurality of depthcontrol foils 25 (which will be explained with reference to FIGS. 2 and3). The depth control foils 25 provide hydrodynamic lift, typically inthe form of downward force on the spreader cables 24 so that thespreader cables 24 and thus the front ends of the streamers (e.g., atterminations 20A) may be submerged to a selected depth in the water 11.

The system shown in FIG. 1, may also include a plurality of LFD controldevices 26 cooperatively engaged with each of the streamers 20 atselected positions along each streamer 20. Each LFD control device 26includes rotatable control surfaces that when moved to a selected rotaryorientation with respect to the direction of movement of such surfacesthrough the water 11 creates a hydrodynamic lift in a selected directionto urge the streamer 20 in any selected direction upward or downward inthe water 11 or transverse to the direction of motion of the vessel.Thus, such LFD control devices 26 can be used to maintain the streamersin a selected geometric arrangement.

An example of one of the depth control foils 25 is shown in oblique viewFIG. 2. The foil 25 may include an opening 25A proximate the forward end25B thereof that will enable coupling the foil 25 on the spreader cable(24 in FIG. 1). The forward end 25B of the foil 25 may be shaped toreduce hydrodynamic drag as the survey system (see FIG. 1) is towedthrough the water (11 in FIG. 1). Each foil 25 may include a curvedupper surface 25C and a tail 25E extending therefrom that extends fromthe upper surface 25C of the foil. The respective lengths of the uppersurface 25C, the tail 25E and the lower surface 25D of the foil 25 areconfigured to generate the desired hydrodynamic force. The foil 25 willthus generate downward force as it is moved through the water byBernoulli's principle. If in certain circumstances it is desired tocreate upward force on the spreader cable (24 in FIG. 1), the foils 25may be mounted on the spreader cable (24 in FIG. 1 in the oppositeconfiguration to generate upward lift.

FIG. 3 shows a plurality of foils 25 disposed side by side on a segmentof spreader cable 24. A hydrodynamic depressor made up of a number ofsmall individual foils 25 as shown in FIG. 3 effectively form a longwing. The foils 25 are can rotate freely around the spreader cable 24for them to be able to adjust to the optimal angle when dragged throughthe water (11 in FIG. 1), and also to be flexible for deck handling andto allow for storage on a winch (e.g., 19 in FIG. 1).

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A hydrodynamic foil system for a sensor streamer array, comprising:at least two tow ropes each coupled at one end to a survey vessel and atthe other end to a paravane, the paravanes configured to provide lateraloutward force as the vessel and paravanes are moved through a body ofwater; a spreader cable coupled to each of the paravanes; a plurality oflaterally spaced apart sensor streamers coupled at forward ends thereofto one of the spreader cables; and a plurality of foils disposed on thespreader cables, the foils configured to provide hydrodynamic lift in avertical direction as the spreader cables are moved through the water.2. The system of claim 1 wherein the vertical direction is downward. 3.The system of claim 1 wherein the foils are each configured to freelyrotate about the spreader cable.
 4. The system of claim 1 whereinsensors on the sensor streamers comprise seismic sensors.
 5. The systemof claim 1 wherein the spreader cables are interconnected between theparavanes.
 6. A method for operating a geophysical data acquisitionsystem, comprising: towing at least two tow ropes from a seismic surveyvessel in a body of water, a distal end of each tow rope having aparavane coupled thereto; maintaining a lateral separation between theparavanes using a spreader cable coupled to each paravane; coupling aforward end of a plurality of sensor streamers at laterally spaced apartpositions to each of the spreader cables; and applying a selectedvertical force to the spreader cables along their length to move thespreader cables to a selected depth in the body of water.
 7. The methodof claim 6 wherein the vertical force is applied by a plurality of foilscoupled to the spreader cables
 8. The method of claim 6 wherein thevertical force is in a downward direction.
 9. The method of claim 6further comprising detecting seismic signals using the streamers.